The snide answer is that only Perl programmers are productive enough to be able to rewrite the foreign code and still get their work done.

But the truth is that there are many who have helped major projects crash and burn by re-inventing the wheel. Check out Crash And Burn Java for instance, among its tips is re-invent your infrastructure so that you can use up some programming talent and then have something else to sell afterwards. (That rant is, I understand, based on an actual project.)